.\"
.\" Copyright (c) 2004-2005
.\"	Hartmut Brandt.
.\"	All rights reserved.
.\" Copyright (c) 2001-2003
.\"	Fraunhofer Institute for Open Communication Systems (FhG Fokus).
.\"	All rights reserved.
.\"
.\" Author: Harti Brandt <harti@FreeBSD.org>
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
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.\"    documentation and/or other materials provided with the distribution.
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.\" $Begemot: bsnmp/lib/bsnmpclient.3,v 1.12 2005/10/04 08:46:50 brandt_h Exp $
.\"
.Dd June 24, 2025
.Dt BSNMPCLIENT 3
.Os
.Sh NAME
.Nm snmp_client ,
.Nm snmp_client_init ,
.Nm snmp_client_set_host ,
.Nm snmp_client_set_port ,
.Nm snmp_send_cb_f ,
.Nm snmp_timeout_cb_f ,
.Nm snmp_timeout_start_f ,
.Nm snmp_timeout_stop_f ,
.Nm snmp_open ,
.Nm snmp_close ,
.Nm snmp_pdu_create ,
.Nm snmp_add_binding ,
.Nm snmp_pdu_check ,
.Nm snmp_pdu_send ,
.Nm snmp_oid_append ,
.Nm snmp_parse_server ,
.Nm snmp_receive ,
.Nm snmp_table_cb_f ,
.Nm snmp_table_fetch ,
.Nm snmp_table_fetch_async ,
.Nm snmp_dialog ,
.Nm snmp_discover_engine
.Nd "SNMP client library"
.Sh LIBRARY
Begemot SNMP library
.Pq libbsnmp, -lbsnmp
.Sh SYNOPSIS
.In asn1.h
.In snmp.h
.In snmpclient.h
.Ft typedef void
.Fn (*snmp_send_cb_f) "struct snmp_pdu *req" "struct snmp_pdu *resp" "void *uarg"
.Ft typedef void
.Fn (*snmp_timeout_cb_f) "void *uarg"
.Ft typedef void *
.Fn (*snmp_timeout_start_f) "struct timeval *timeout" "snmp_timeout_cb_f callback" "void *uarg"
.Ft typedef void
.Fn (*snmp_timeout_stop_f) "void *timeout_id"
.Vt extern struct snmp_client snmp_client ;
.Ft void
.Fn snmp_client_init "struct snmp_client *client"
.Ft int
.Fn snmp_client_set_host "struct snmp_client *client" "const char *host"
.Ft int
.Fn snmp_client_set_port "struct snmp_client *client" "const char *port"
.Ft int
.Fn snmp_open "const char *host" "const char *port" "const char *read_community" "const char *write_community"
.Ft void
.Fn snmp_close "void"
.Ft void
.Fn snmp_pdu_create "struct snmp_pdu *pdu" "u_int op"
.Ft int
.Fn snmp_add_binding "struct snmp_pdu *pdu" "..."
.Ft int
.Fn snmp_pdu_check "const struct snmp_pdu *req" "const struct snmp_pdu *resp"
.Ft int32_t
.Fn snmp_pdu_send "struct snmp_pdu *pdu" "snmp_send_cb_f func" "void *uarg"
.Ft int
.Fn snmp_oid_append "struct asn_oid *oid" "const char *fmt" "..."
.Ft int
.Fn snmp_parse_server "struct snmp_client *sc" "const char *str"
.Ft int
.Fn snmp_receive "int blocking"
.Ft typedef void
.Fn (*snmp_table_cb_f) "void *list" "void *arg" "int res"
.Ft int
.Fn snmp_table_fetch "const struct snmp_table *descr" "void *list"
.Ft int
.Fn snmp_table_fetch_async "const struct snmp_table *descr" "void *list" "snmp_table_cb_f callback" "void *uarg"
.Ft int
.Fn snmp_dialog "struct snmp_pdu *req" "struct snmp_pdu *resp"
.Ft int
.Fn snmp_discover_engine "void"
.Sh DESCRIPTION
The SNMP library contains routines to easily build SNMP client applications
that use SNMP versions 1, 2 or 3.
Most of the routines use a
.Vt struct snmp_client :
.Bd -literal -offset indent
struct snmp_client {
	enum snmp_version	version;
	int			trans;	/* which transport to use */

	/* these two are read-only for the application */
	char			*cport;	/* port number as string */
	char			*chost;	/* host name or IP address as string */

	char			read_community[SNMP_COMMUNITY_MAXLEN + 1];
	char			write_community[SNMP_COMMUNITY_MAXLEN + 1];

	/* SNMPv3 specific fields */
	int32_t			identifier;
	int32_t			security_model;
	struct snmp_engine	engine;
	struct snmp_user	user;

	/* SNMPv3 Access control - VACM*/
	uint32_t		clen;
	uint8_t			cengine[SNMP_ENGINE_ID_SIZ];
	char			cname[SNMP_CONTEXT_NAME_SIZ];

	struct timeval		timeout;
	u_int			retries;

	int			dump_pdus;

	size_t			txbuflen;
	size_t			rxbuflen;

	int			fd;

	int32_t			next_reqid;
	int32_t			max_reqid;
	int32_t			min_reqid;

	char			error[SNMP_STRERROR_LEN];

	snmp_timeout_start_f	timeout_start;
	snmp_timeout_stop_f	timeout_stop;

	char			local_path[SUNPATHLEN];
};
.Ed
.Pp
The fields of this structure are described below.
.Bl -tag -width "timeout_start"
.It Va version
This is the version of SNMP to use.
See
.Xr bsnmplib 3
for applicable values.
The default version is
.Li SNMP_V2c .
.It Va trans
If this is
.Dv SNMP_TRANS_LOC_DGRAM
a local datagram socket is used.
If it is
.Dv SNMP_TRANS_LOC_STREAM
a local stream socket is used.
For
.Dv SNMP_TRANS_UDP
a UDPv4 socket and for
.Dv SNMP_TRANS_UDP6
a UDPv6 socket is created.
It uses the
.Va chost
field as the path to the server's socket for local sockets.
.It Va cport
The SNMP agent's UDP port number.
This may be a symbolic port number (from
.Pa /etc/services )
or a numeric port number.
If this field is
.Li NULL
(the default) the standard SNMP port is used.
This field should not be changed directly but rather by calling
.Fn snmp_client_set_port .
.It Va chost
The SNMP agent's host name, IP address or
.Ux
domain socket path name.
If this is
.Li NULL
(the default)
.Li localhost
is assumed.
This field should not be changed directly but rather through calling
.Fn snmp_client_set_host .
.It Va read_community
This is the community name to be used for all requests except SET requests.
The default is
.Sq public .
.It Va write_community
The community name to be used for SET requests.
The default is
.Sq private .
.It Va identifier
The message identifier value to be used with SNMPv3 PDUs. Incremented with
each transmitted PDU.
.It Va security_model
The security model to be used with SNMPv3 PDUs. Currently only User-Based
Security model specified by RFC 3414 (value 3) is supported.
.It Va engine
The authoritive SNMP engine parameters to be used with SNMPv3 PDUs.
.It Va user
The USM SNMP user credentials to be used with SNMPv3 PDUs.
.It Va clen
The length of the context engine id to be used with SNMPv3 PDUs.
.It Va cengine
The context engine id to be used with SNMPv3 PDUs. Default is empty.
.It Va cname
The context name to be used with SNMPv3 PDUs. Default is
.Sq "" .
.It Va timeout
The maximum time to wait for responses to requests.
If the time elapses, the request is resent up to
.Va retries
times.
The default is 3 seconds.
.It Va retries
Number of times a request PDU is to be resent.
If set to 0, the request is sent only once.
The default is 3 retransmissions.
.It Va dump_pdus
If set to a non-zero value all received and sent PDUs are dumped via
.Xr snmp_pdu_dump 3 .
The default is not to dump PDUs.
.It Va txbuflen
The encoding buffer size to be allocated for transmitted PDUs.
The default is 10000 octets.
.It Va rxbuflen
The decoding buffer size to be allocated for received PDUs.
This is the size of the maximum PDU that can be received.
The default is 10000 octets.
.It Va fd
After calling
.Fn snmp_open
this is the file socket file descriptor used for sending and receiving PDUs.
.It Va next_reqid
The request id of the next PDU to send.
Used internal by the library.
.It Va max_reqid
The maximum request id to use for outgoing PDUs.
The default is
.Li INT32_MAX .
.It Va min_reqid
The minimum request id to use for outgoing PDUs.
Request ids are allocated linearily starting at
.Va min_reqid
up to
.Va max_reqid .
.It Va error
If an error happens, this field is set to a printable string describing the
error.
.It Va timeout_start
This field must point to a function setting up a one shot timeout.
After the timeout has elapsed, the given callback function must be called
with the user argument.
The
.Fn timeout_start
function must return a
.Vt void *
identifying the timeout.
.It Va timeout_stop
This field must be set to a function that stops a running timeout.
The function will be called with the return value of the corresponding
.Fn timeout_start
function.
.It Va local_path
In local socket mode, optional path name the client socket shall be bound to
before connecting to the server.
For
.Dv SOCK_STREAM
local socket the named path is optional, and library will skip
.Xr bind 2
if path is not provided.
For
.Dv SOCK_DGRAM
local socket the named path is required, thus library will create a random
one in
.Pa /tmp
if path is not provided.
.El
.Pp
In the current implementation there is a global variable
.Pp
.D1 Vt extern struct snmp_client snmp_client ;
.Pp
that is used by all the library functions.
The first call into the library must be a call to
.Fn snmp_client_init
to initialize this global variable to the default values.
After this call and before calling
.Fn snmp_open
the fields of the variable may be modified by the user.
The modification of the
.Va chost
and
.Va cport
fields should be done only via the functions
.Fn snmp_client_set_host
and
.Fn snmp_client_set_port .
.Pp
The function
.Fn snmp_open
creates a UDP or
.Ux
domain socket and connects it to the agent's IP address and port.
If any of the arguments of the call is not
.Li NULL
the corresponding field in the global
.Va snmp_client
is set from the argument.
Otherwise the values that are already in that variable are used.
The function
.Fn snmp_close
closes the socket, stops all timeouts and frees all dynamically allocated
resources.
.Pp
The next three functions are used to create request PDUs.
The function
.Fn snmp_pdu_create
initializes a PDU of type
.Va op .
It does not allocate space for the PDU itself.
This is the responsibility of the caller.
.Fn snmp_add_binding
adds bindings to the PDU and returns the (zero based) index of the first new
binding.
The arguments are pairs of pointer to the OIDs and syntax constants,
terminated by a NULL.
The call
.Bd -literal -offset indent
snmp_add_binding(&pdu,
    &oid1, SNMP_SYNTAX_INTEGER,
    &oid2, SNMP_SYNTAX_OCTETSTRING,
    NULL);
.Ed
.Pp
adds two new bindings to the PDU and returns the index of the first one.
It is the responsibility of the caller to set the value part of the binding
if necessary.
The functions returns -1 if the maximum number of bindings is exhausted.
The function
.Fn snmp_oid_append
can be used to construct variable OIDs for requests.
It takes a pointer to an
.Vt struct asn_oid
that is to be constructed, a format string, and a number of arguments
the type of which depends on the format string.
The format string is interpreted
character by character in the following way:
.Bl -tag -width ".It Li ( Va N Ns Li )"
.It Li i
This format expects an argument of type
.Vt asn_subid_t
and appends this as a single integer to the OID.
.It Li a
This format expects an argument of type
.Vt struct in_addr
and appends to four parts of the IP address to the OID.
.It Li s
This format expects an argument of type
.Vt const char *
and appends the length of the string (as computed by
.Xr strlen 3 )
and each of the characters in the string to the OID.
.It ( Va N Ns )
This format expects no argument.
.Va N
must be a decimal number and is stored into an internal variable
.Va size .
.It Li b
This format expects an argument of type
.Vt const char *
and appends
.Va size
characters from the string to the OID.
The string may contain
.Li NUL
characters.
.It Li c
This format expects two arguments: one of type
.Vt size_t
and one of type
.Vt const u_char * .
The first argument gives the number of bytes to append to the OID from the string
pointed to by the second argument.
.El
.Pp
The function
.Fn snmp_pdu_check
may be used to check a response PDU.
A number of checks are performed
(error code, equal number of bindings, syntaxes and values for SET PDUs).
The function returns +1 if everything is ok, 0 if a NOSUCHNAME or similar
error was detected, -1 if the response PDU had fatal errors
and -2 if
.Fa resp
is
.Li NULL
(a timeout occurred).
.Pp
The function
.Fn snmp_pdu_send
encodes and sends the given PDU.
It records the PDU together with the callback
and user pointers in an internal list and arranges for retransmission if no
response is received.
When a response is received or the retransmission count
is exceeded the callback
.Fa func
is called with the original request PDU, the response PDU and the user argument
.Fa uarg .
If the retransmit count is exceeded,
.Fa func
is called with the original request PDU, the response pointer set to
.Li NULL
and the user argument
.Fa uarg .
The caller should not free the request PDU until the callback function is
called.
The callback function must free the request PDU and the response PDU (if not
.Li NULL ).
.Pp
The function
.Fn snmp_receive
tries to receive a PDU.
If the argument is zero, the function polls to see
whether a packet is available, if the argument is non-zero, the function blocks
until the next packet is received.
The packet is delivered via the usual callback
mechanism (non-response packets are silently dropped).
The function returns 0, if a packet was received and successfully dispatched,
-1 if an error occurred or no packet was available (in polling mode).
.Pp
The next two functions are used to retrieve tables from SNMP agents.
They use
the following input structure, that describes the table:
.Bd -literal -offset indent
struct snmp_table {
	struct asn_oid		table;
	struct asn_oid		last_change;
	u_int			max_iter;
	size_t			entry_size;
	u_int			index_size;
	uint64_t		req_mask;

	struct snmp_table_entry {
	    asn_subid_t		subid;
	    enum snmp_syntax	syntax;
	    off_t		offset;
	}			entries[];
};
.Ed
.Pp
The fields of this structure have the following meaning:
.Bl -tag -width "last_change"
.It Va table
This is the base OID of the table.
.It Va last_change
Some tables have a scalar variable of type TIMETICKS attached to them,
that holds the time when the table was last changed.
This OID should be the OID of this variable (without the \&.0 index).
When the table is retrieved
with multiple GET requests, and the variable changes between two request,
the table fetch is restarted.
.It Va max_iter
Maximum number of tries to fetch the table.
.It Va entry_size
The table fetching routines return a list of structures one for each table
row.
This variable is the size of one structure and used to
.Xr malloc 3
the structure.
.It Va index_size
This is the number of index columns in the table.
.It Va req_mask
This is a bit mask with a 1 for each table column that is required.
Bit 0 corresponds to the first element (index 0) in the array
.Va entries ,
bit 1 to the second (index 1) and so on.
SNMP tables may be sparse.
For sparse columns the bit should not be set.
If the bit for a given column is set and
the column value cannot be retrieved for a given row, the table fetch is
restarted assuming that the table is currently being modified by the agent.
The bits for the index columns are ignored.
.It Va entries
This is a variable sized array of column descriptors.
This array is terminated by an element with syntax
.Li SNMP_SYNTAX_NULL .
The first
.Va index_size
elements describe all the index columns of the table, the rest are normal
columns.
If for the column at
.Ql entries[N]
the expression
.Ql req_mask & (1 << N)
yields true, the column is considered a required column.
The fields of this the array elements have the following meaning:
.Bl -tag -width "syntax"
.It Va subid
This is the OID subid of the column.
This is ignored for index entries.
Index entries are decoded according to the
.Va syntax
field.
.It Va syntax
This is the syntax of the column or index.
A syntax of
.Li SNMP_SYNTAX_NULL
terminates the array.
.It Va offset
This is the starting offset of the value of the column in the return structures.
This field can be set with the ISO-C
.Fn offsetof
macro.
.El
.El
.Pp
Both table fetching functions return TAILQ (see
.Xr queue 3 )
of structures--one for each table row.
These structures must start with a
.Fn TAILQ_ENTRY
and a
.Vt uint64_t
and are allocated via
.Xr malloc 3 .
The
.Fa list
argument of the table functions must point to a
.Fn TAILQ_HEAD .
The
.Vt uint64_t
fields, usually called
.Va found
is used to indicate which of the columns have been found for the given
row.
It is encoded like the
.Fa req_mask
field.
.Pp
The function
.Fn snmp_table_fetch
synchronously fetches the given table.
If everything is ok 0 is returned.
Otherwise the function returns -1 and sets an appropriate error string.
The function
.Fn snmp_table_fetch_async
fetches the tables asynchronously.
If either the entire table is fetch, or
an error occurs the callback function
.Fa callback
is called with the callers arguments
.Fa list
and
.Fa uarg
and a parameter that is either 0 if the table was fetched, or
-1 if there was an error.
The function itself returns -1 if it could not
initialize fetching of the table.
.Pp
The following table description is used to fetch the ATM interface table:
.Bd -literal -offset indent
/*
 * ATM interface table
 */
struct atmif {
	TAILQ_ENTRY(atmif) link;
	uint64_t	found;
	int32_t		index;
	u_char		*ifname;
	size_t		ifnamelen;
	uint32_t	node_id;
	uint32_t	pcr;
	int32_t		media;
	uint32_t	vpi_bits;
	uint32_t	vci_bits;
	uint32_t	max_vpcs;
	uint32_t	max_vccs;
	u_char		*esi;
	size_t		esilen;
	int32_t		carrier;
};
TAILQ_HEAD(atmif_list, atmif);

/* list of all ATM interfaces */
struct atmif_list atmif_list;

static const struct snmp_table atmif_table = {
	OIDX_begemotAtmIfTable,
	OIDX_begemotAtmIfTableLastChange, 2,
	sizeof(struct atmif),
	1, 0x7ffULL,
	{
	  { 0, SNMP_SYNTAX_INTEGER,
		offsetof(struct atmif, index) },
	  { 1, SNMP_SYNTAX_OCTETSTRING,
		offsetof(struct atmif, ifname) },
	  { 2, SNMP_SYNTAX_GAUGE,
		offsetof(struct atmif, node_id) },
	  { 3, SNMP_SYNTAX_GAUGE,
		offsetof(struct atmif, pcr) },
	  { 4, SNMP_SYNTAX_INTEGER,
		offsetof(struct atmif, media) },
	  { 5, SNMP_SYNTAX_GAUGE,
		offsetof(struct atmif, vpi_bits) },
	  { 6, SNMP_SYNTAX_GAUGE,
		offsetof(struct atmif, vci_bits) },
	  { 7, SNMP_SYNTAX_GAUGE,
		offsetof(struct atmif, max_vpcs) },
	  { 8, SNMP_SYNTAX_GAUGE,
		offsetof(struct atmif, max_vccs) },
	  { 9, SNMP_SYNTAX_OCTETSTRING,
		offsetof(struct atmif, esi) },
	  { 10, SNMP_SYNTAX_INTEGER,
		offsetof(struct atmif, carrier) },
          { 0, SNMP_SYNTAX_NULL, 0 }
	}
};

\&...
	if (snmp_table_fetch(&atmif_table, &atmif_list) != 0)
		errx(1, "AtmIf table: %s", snmp_client.error);
\&...
.Ed
.Pp
The function
.Fn snmp_dialog
is used to execute a synchonuous dialog with the agent.
The request PDU
.Fa req
is sent and the function blocks until the response PDU is received.
Note,
that asynchonuous receives are handled (i.e. callback functions of other send
calls or table fetches may be called while in the function).
The response PDU is returned in
.Fa resp .
If no response could be received after all timeouts and retries, the function
returns -1.
If a response was received 0 is returned.
.Pp
The function
.Fn snmp_discover_engine
is used to discover the authoritative snmpEngineId of a remote SNMPv3 agent.
A request PDU with empty USM user name is sent and the client's engine
parameters are set according to the snmpEngine parameters received in the
response PDU.
If the client is configured to use authentication and/or privacy and the
snmpEngineBoots and/or snmpEngineTime in the response had zero values, an
additional request (possibly encrypted) with the appropriate user credentials
is sent to fetch the missing values.
Note, that the function blocks until the discovery process is completed.
If no response could be received after all timeouts and retries, or the
response contained errors the function returns -1.
If the discovery process was completed 0 is returned.
.Pp
The function
.Fn snmp_parse_server
is used to parse an SNMP server specification string and fill in the
fields of a
.Vt struct snmp_client .
The syntax of a server specification is
.Pp
.D1 [trans::][community@][server][:port]
.Pp
where
.Va trans
is the transport name (one of
.Qq udp ,
.Qq udp6 ,
.Qq stream
or
.Qq dgram ) ,
.Va community
is the string to be used for both the read and the write community,
.Va server
is the server's host name in case of UDP and the path name in case
of a local socket, and
.Va port
is the port in case of UDP transport.
The function returns 0 in the case of success and return -1 and sets
the error string in case of an error.
.Pp
The function
.Fn snmp_parse_serverr
fills the transport, the port number and the community strings with
reasonable default values when they are not specified.
The default transport
is
.Dv SNMP_TRANS_UDP .
If the host name contains a slash the default is modified to
.Dv SNMP_TRANS_LOC_DGRAM .
If the host name looks like a numeric IPv6 address the default is
.Dv SNMP_TRANS_UDP6 .
For numeric IPv6 addresses the transport name udp is automatically
translated as
.Dv SNMP_TRANS_UDP6 .
The default port number (for
.Dv udp
or
.Dv udp6 )
is
.Qq snmp .
The default read community is
.Qq public
and the default write community
.Qq private .
.Pp
.Fn snmp_parse_server
recognizes path names, host names and numerical IPv4 and IPv6 addresses.
A string consisting of digits and periods is assumed to be an IPv4 address
and must be parseable by
.Fn inet_aton 3 .
An IPv6 address is any string enclosed in square brackets.
It must be parseable with
.Fn gethostinfo 3 .
.Pp
The port number for
.Fn snmp_parse_server
can be specified numerically or symbolically.
It is ignored for local sockets.
.Sh DIAGNOSTICS
If an error occurs in any of the functions an error indication as described
above is returned.
Additionally the function sets a printable error string in the
.Va error
field of
.Va snmp_client .
.Sh SEE ALSO
.Xr gensnmptree 1 ,
.Xr bsnmpd 1 ,
.Xr bsnmpagent 3 ,
.Xr bsnmplib 3
.Sh STANDARDS
This implementation conforms to the applicable IETF RFCs and ITU-T
recommendations.
.Sh AUTHORS
.An Hartmut Brandt Aq harti@FreeBSD.org
.An Kendy Kutzner Aq kutzner@fokus.gmd.de
